The present invention relates to a process for the production of citraconic anhydride, particularly by oxidation of isoprene.
Citraconic acid anhydride is a known chemical having known uses such as a food acidulant and in preparing polymers or resins, which resins can be used in production of fiberglass reinforced plastics.
Previous methods for producing citraconic anhydride have included the distillation of citric acid. Rapid distillation of citric acid yields citraconic and itaconic anhydride. Repeated distillation of itaconic anhydride yields citraconic anhydride. See, for example, Kirk-Othmer, Encyclopedia of Chemical Technology, Second Edition, Volume 1, at page 253; U.S. Pat. No. 2,088,347; and U.S. Pat. No. 2,258,947.
U.S. Pat. No. 2,966,498 discloses preparation of citraconic anhydride by pyrolysis of itaconic acid in the presence of alkali metal salts.
Several patents to Kerr, namely 3,296,282; 3,255,211; 3,255,212; 3,255,213; 3,156,705; 3,156,706; 3,156,707; and 3,094,539, discloses oxidation of olefinic feeds to dicarboxylic acid anhydrides, particularly maleic anhydride. Although the Kerr patents are concerned with the oxidation of butene-2, other ethylenically unsaturated feeds are mentioned, such as 3-methylbutene-1, isoprene, 2,3-dimethyl butadiene and butadiene-1,3 and mixtures thereof. The Kerr references disclose numerous hydrocarbon oxidation catalysts, including vanadium and phosphorus mixed oxide catalyst, which catalyst may contain added alkali or alkaline earth components as disclosed, for example, in U.S. Pat. Nos. 3,255,211 and 3,255,212.
U.S. Pat. Nos. 3,478,063 and 3,538,122 to Friedrichsen et al also disclose hydrocarbon oxidation catalysts for converting unsaturated hydrocarbons to maleic anhydride.
U.S. Pat. No. 2,719,853 discloses the catalytic production of citraconic acid by oxidation of a hydrocarbon. According to this patent, the catalyst used in a vanadium oxide catalyst promoted with arsenic. The hydrocarbon feed used in the examples of this patent was a cracked naphtha containing C.sub.4 -C.sub.6 .sup.+ hydrocarbons, including paraffins and olefins. No iso-C.sub.5 olefins were disclosed as present in the feed, and the feed was evidently passed directly over the vanadium arsenate catalyst at 900.degree.F with no pretreatment.